High purity products are conventionally shipped in bulk in hopper cars whose housing includes an upwardly extending loading member defining therewithin a large cross-sectional loading aperture which communicates with the interior of the hopper car. Such hopper cars are provided with a hatch cover fittable over the loading member in a closed position covering the loading aperture. While such arrangement adequately protects many kinds of shipments during shipping, for high purity products, excessive amounts of dirt and other contamination enter through such an aperture. It is conventional to counteract this problem by placing a flexible plastic sheet or the like over the top of the loading member and beneath the hatch. Typically, such flexible sheet is fastened by an elastic member or the like pressing the flexible plastic sheet against the outside of the loading member and causing it to gusset and assume the general appearance of a shower cap.
Such a "shower cap" arrangement has the disadvantage that dirt and other contamination tend to accumulate on the top of the shower cap during loading and shipment such that, upon unloading, great care must be taken to prevent such contamination from falling into the hopper car through the loading aperture. This can occur when the flexible plastic sheet tears or when, in removing the flexible plastic sheet, dirt is permitted to fall from an edge of the flexible plastic sheet into the loading aperture. Since such apertures typically are of relatively large cross-sectional area, over a foot in diameter, such contamination of a shipment can occur in the absence of special care.